Shape Definition
Through Shape we are able to discern the forms that make up our world. All visible objects have shape. A shape is a distinct area that is visually separated from the picture plane and the other elements of the design.
Closure
Shapes, like lines, can be implied. In the case of a partial or broken line, humans will naturally fill in the shape. This phenomenon is called Closure.
Geometric and Organic Shapes
Geometric shapes are usually easily mathematically defined and symmetrical. They have a static, stable feel.
Organic shapes are usually rounded, irregular, and free flowing. They tend to be more visually complex. They usually have a dynamic feel. (Vivian from Paper Mario)